Conventional state of the art uses packing glands as seals for the spindles or arbors of shut-off fittings. As the spindles or arbors turn these seals are subject to high wear, attributable essentially to the dynamic stress of the sealing elements. Other known valve fittings with spindle back seals (conus, labyrinth boxes) are effective only if the valve is opened completely, in which case they constitute an additional static seal. The purpose of this type of seal is in effect to offer the possibility of replacing worn packing glands. Solutions with conventional packing glands do not achieve the sealing quality required under the legal regulations valid in Germany today.
Another known technique is to equip spindle and arbor seals with a bellow-type valve. So far this design has been used only with small nominal diameters (under DN 200). The bellow-type design requires an overall height which is three to four times the diameter of the pipe and thus is unsuitable for many applications, due to a lack of space. In addition, the bellow-type design has the shortcoming that a defective bellow requires the replacement of the entire shut-off fitting.
The shut-off fitting described in the U.S. Pat. No. 2,780,233 also has the disadvantage that it has a relatively large overall height. Neither does the use of a single packing gland, which in this design serves as the main seal in different operating positions, meet the legal requirements that apply in Germany. In particular, compliance with the corresponding sealing requirements cannot be guaranteed during the required replacement of packing glands, because this design lacks a back sealing element. Finally, the design of the solution proposed in U.S. Pat. No. 2,780,233 is relatively complex. This makes it necessary to disassemble and re-assemble a number of parts of the housing when the packing gland is changed.